Fluidflow systems are used in many applications to provide heating and cooling to buildings and equipment. One of the most common forms is by forced air moved by fans. It is useful and often necessary to monitor the airflow (or other fluidflow) in order to provide a signal if the airflow ceases or drops below a predetermined level. Traditionally, a number of types of airflow switches or sensors have been made, including vane switches and heated thermostat switches. In vane switches a vane deflected by normal airflow returns to its initial position as the flow decreases until a contact on the vane engages a fixed contact and closes a certain circuit. Such switches are subject to failure due to dirt in the bearings and dirt on the contacts, intermittent contact closure, contact bounce, imprecise calibration and unpredicted contact action due to random vibration and other causes. Heated thermostat switches also are subject to dirt buildup on contacts and have very slow reaction times. Both are subject to contact deterioration.
More recently, two so-called solid state switches have been used. One type employs a self-heated temperature-sensitive resistance (thermistor) whose temperature remains constant with constant airflow. Upon decrease of the flow there is less heat dissipation and the temperature rises, causing a lowering of resistance. When the condition deteriorates sufficiently the resistance reduction reaches a pre-established value, which functions as a trip point and is detected as an alarm condition. In another type, using a self-heated temperature-sensitive resistance (posistor), upon decrease of the flow there is less heat dissipation and the temperature rises, causing the resistance to approach a knee value where the resistance abruptly changes. When the conditions deteriorate sufficiently for the knee value to be reached it changes abruptly and functions as a trip point, which is detected as an alarm condition. The use of the area of abrupt change or the "knee" of the characteristic limits the range of trip points selectable for operation and may introduce a lack of precision into the detection process. In addition, these devices are bidirectionally sensitive: equally sensitive to flow variations in the forward and the reverse direction. This bidirectionality is problematic: in many cases a duct, plenum or other area of airflow is served by a number of fans. If one fan fails the increased pressure and flow in the area generates a reverse flow through that stopped fan and any associated airflow switch, thereby maintaining a false indication of proper flow.